The present invention relates to an improved knockout punch which is used in conjunction with a punch driver to punch holes in sheet metal, for example, in the walls of electrical cabinets, aluminum, fiberglass and plastic.
Generally, when a hole is to be punched in an electrical cabinet, a small hole is first drilled in the wall of the electrical cabinet. A first end of a draw stud is threaded into a ram of a hydraulic punch driver. A second end of the draw stud is inserted through a punching die and then through the drilled hole, the draw stud having a circumference that is less than the circumference of the drilled hole. A knockout punch is threaded onto the second end of the draw stud on the opposite side of the electrical cabinet than is the punching die and the hydraulic punch driver.
An operator actuates a hand pump of the hydraulic punch driver. When the hand pump of the hydraulic punch driver is actuated, hydraulic fluid forces the ram to pull the draw stud. The draw stud, in turn, pulls the knockout punch through the electrical cabinet into the die such that the desired hole size is punched.
Knockout punches used in the prior art, such as standard round knockout punches, SLUG BUSTER® knockout punches sold by Greenlee Textron Inc., the assignee of the present invention, and those embodied in U.S. Pat. No. 4,353,164, which is owned by Greenlee Textron Inc., the assignee of the present invention, while proving very effective in the marketplace, suffer from a number of disadvantages.
One such disadvantage is that the prior art knockout punches do not provide means for locating the punch assembly in a pilot hole as the knockout punch and the die are drawn together by the draw stud to make a hole in the workpiece. Presently, an operator locates the punch assembly in a pilot hole by using “alignment marks” which can be difficult to see by the operator and may allow for error such that the hole to be created may not be properly positioned.
Another such disadvantage is that the prior art knockout punches typically have an elevated punching force at the beginning as the punch pierces the workpiece because the punch is working against a large length of the workpiece before the points of the punch fully pass through the workpiece.
Yet another such disadvantage is that the prior art knockout punches typically have an elevated punching force at the end of the punching cycle. The standard punch has a high punching force at the end of the punching cycle because it is shearing on four lines simultaneously and the angle of the punch faces reduces to zero at the end. The SLUG BUSTER® punch also has a high punching force at the end of the punching cycle because the long angled punch surfaces are “v” shaped, shearing on four lines simultaneously as the angle of the punch face reduces to zero toward the end of the punching cycle.
Another such disadvantage of the prior art knockout punches is that they do not have only planar surfaces which can be machined with standard cutting tools, such that custom formed tools or broaches are required to form the prior art knockout punches.
Thus, it is desirable to have a knockout punch which incorporates all of the advantages of the prior art knockout punches, but which overcomes the disadvantages of the prior art knockout punches, such as those identified above. The invention, as described herein, provides such a knockout punch. Other features and advantages of the knockout punch of the present invention will become apparent upon a reading of the attached specification in combination with a study of the drawings.